1. Field of the Invention:
The present invention relates to polysiloxane compositions which are particularly suitable for the production of peelable coatings, and very especially suitable for transfer adhesives in which the adhesive layer and the nonstick layer are both silicone formulations.
2. Description of the Prior Art:
Nonstick coatings can be used in all cases where it is necessary to produce a surface or a material which is relatively nonadhesive to other materials to which it would normally adhere. Nonstick silicone compositions are widely employed for papers as coatings which are removed from contact adhesives, for labels, decorative laminates, adhesive transfer tapes, and the like. Nonstick silicone coatings for paper, polyethylene, Mylar and other substrates of such type can also be used as nontacky surfaces for applications in the handling of food products and in industrial packaging. These coatings are normally in a solvent-free state, in the form of a solution in a solvent, or in the form of an aqueous emulsion.
When labels are coated with an adhesive, it is desirable that the paper backing be easily removed from the label when it is about to be used, without the adhesive nature of the label being altered by its separation from the substrate on which it has been stored. The same principle applies to certain types of adhesive tapes in roll form. The tape has to unwind easily while retaining its adhesive properties. This can be achieved by covering the nonadhesive side of the tape with a peelable nonstick silicone composition which will be placed in contact with the adhesive when the roll of tape is manufactured.
Nonstick coatings can be employed in combination with silicone-based adhesives to form a system of transfer adhesives. An adhesive of this type can have many applications such as, for example, the restoration of works of art, as described in detail in French Patent No. FR-A-2,478,657 and European Patent No. EP-A-33,193. In an adhesive of this type, a first substrate, normally consisting of any type of nonporous film or of glazed paper, and preferably supercalendered kraft paper, is covered with a nonstick silicone coating, which can be removed from a reactive silicone-based contact adhesive. These two compositions will be described hereinbelow. The nonstick coating is cured on the first substrate by well known means, and normally by heat. The silicone-based contact adhesive is then applied onto the nonstick coating and is cured in place, such that the first substrate now bears two coatings, namely, the nonstick silicone coating and the silicone adhesive.
The first substrate bearing the twin coating is then applied onto a second substrate which is suitable for reinforcement and for the restoration of the work of art, such as to define a laminate. When this stage of forming a laminate has been completed, a transfer adhesive system is available, and this can be employed to reinforce the work of art whenever this may be desired in the future. The restoration is carried out by separating the first substrate and its nonstick coating from the second substrate, which retains the silicone-based contact adhesive, since the reactive adhesive adheres much more strongly to the second substrate than to the nonstick coating with which it was in contact. The silicone adhesive has thus been transferred from the substrate bearing the nonstick coating and there results a second reinforcing substrate on the work of art to constitute a laminate.
A transfer adhesive system of this type is quite obviously not limited to the restoration of paintings, but can be employed whenever it is desirable to apply a completely cured contact adhesive to virtually any sort of substrate. Once the adhesive has been transferred, it is immediately ready for use if moderate pressures are applied.
These transfer adhesive systems based on silicones have the further advantage of remaining flexible over a wide temperature range. This flexibility is not present in organic adhesives such as acrylic substances or epoxy resins, which form inefficient adhesives when applied to a substrate which normally is subjected to relatively high or low temperatures. For example, a transfer adhesive system based on silicone having a high content of phenyl groups will be particularly useful when it is desired to apply an adhesive to a motor. Organic adhesives will flake off, will split or will not satisfactorily adhere to a substrate of this type. However, the silicone adhesive can be deposited onto its transfer substrate bearing a nonstick coating and may be cured thereon, after which it can be later transferred onto the hot section of the motor, which will retain the adhesive. A silicone-based contact adhesive can withstand the temperatures, normally high, to which a motor may be subjected. A label or another laminate can subsequently be applied onto this adhesive.
In a transfer adhesive of this type, the technical problem is extremely difficult to solve.
The nonstick coating and the adhesive must, in fact, adhere sufficiently to each other to ensure the coherence of the laminate. This adhesiveness must obviously remain moderate such that the adhesive may be easily and completely transferred when the laminate is employed. In addition, this adhesive, once separated from the nonstick silicone coating, must adhere sufficiently to the substrate onto which it is transferred.
This substrate may be of any nature and may in particular be a silicone elastomer containing a medication in the case of the transdermal administration of such medication. The silicone elastomer which is thus adhesive-treated must exhibit good adhesion to the patient's skin.